Using "#define" in a Header in C

Question

I'm confused how to use #define, or if this is even what I want to be using.

I have a header file:

#define var     3;

int foo();

...(etc)

As well as two similarly structured files outside of it:

#include "header.h"

int main(int argc, char **argv){

    printf("%i", var);
}

Am I wrong in thinking that I can use the var from the header in C files which include the header?

This is actually a part of homework, and I'm not permitted to change the header file. If I can't use it like this, is there some way to use the variable outside the file other than an accessor function?

#define var 3; is horrible, horrible C (especially that semicolon on the end). If this is the code you were provided, you should complain to the instructor... — nneonneo
– nneonneo, Commented Mar 3, 2014 at 23:21
But your header file is strange. The ; after 3 should not be there. — timrau
– timrau, Commented Mar 3, 2014 at 23:22

pasztorpisti · Accepted Answer · 2014-03-03 23:25:53Z

3

Just don't put a semicolon to the end of your defined value. Instead of

#define var 3;

use

#define var 3

With your define the print statement is expanded to the following by the preprocessor:

printf("%i", 3;);

...that is invalid code. The preprocessor doesn't really respect the rules of the C/C++ language, it's rather a barbar textual substitution tool, maybe a bit smarter than that but much more dumber than the C language itself...

If you are not allowed the change the header then you can still say

int my_variable = var

And you can even put any number of semicolons after the previous statement if you like that more.

answered Mar 3, 2014 at 23:25

pasztorpisti

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John · Accepted Answer · 2014-03-03 23:23:37Z

1

The header file needs to be properly defined:

#ifndef HEADER_FILE
#define HEADER_FILE

#define var 3 // <-- no semi

#endif

Now depending on how you defined var, you should be able to do:

#include "header.h"

int main(int argc, char **argv){

    printf("%i", var);
}

answered Mar 3, 2014 at 23:23

John

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